在Angular应用中发送自定义cookie，需要将自定义cookie作为HttpHeaders传递给HttpClient的方法中。以下是具体实现步骤：

1. 在服务中，将自定义cookie存储在local storage中，以便在需要时获取。

import { Injectable } from '@angular/core';

@Injectable({
  providedIn: 'root'
})
export class CookieService {
  setCookie(name: string, value: string, expireDays: number) {
    const d = new Date();
    d.setTime(d.getTime() + expireDays * 24 * 60 * 60 * 1000);
    const expires = 'expires=' + d.toUTCString();
    document.cookie = name + '=' + value + '; ' + expires;
  }

  getCookie(name: string) {
    const cookies = document.cookie.split('; ');
    for (let i = 0; i < cookies.length; i++) {
      const cookie = cookies[i].split('=');
      if (cookie[0] === name) {
        return cookie[1];
      }
    }
    return '';
  }
}

2. 在需要发送请求的组件中获取cookie，并将其包含在http header中发送。

import { Component } from '@angular/core';
import { HttpHeaders, HttpClient } from '@angular/common/http';
import { CookieService } from '../cookie.service';

@Component({
  selector: 'app-root',
  templateUrl: './app.component.html',
  styleUrls: ['./app.component.scss']
})
export class AppComponent {
  title = 'my-app';

  constructor(private http: HttpClient, private cookieService: CookieService) { }

  sendRequest() {
    const token = this.cookieService.getCookie('token');
    const headers = new HttpHeaders().set('Authorization', `Bearer ${token}`);
    const url = 'https://api.example.com/data';
    this.http.get(url, { headers }).subscribe((response) => {
      console.log(response);
    });
  }
}

以上代码示例演示了如何获取自定义cookie并将其包含在http header中发送。简而言之，需要使用Angular HttpInterceptor机制，在intercept()函数中处理请求头中加入已存储自定义cookie。